legacycentral bbs

When I first installed my Saab IC and MBC, the gauge was reading correctly (as far as I know, it is Autometer) but lately something's been acting up. As soon as it got cold I was hitting boost cut but the gauge was only displaying 6 psi. I backed off the MBC a tiny bit and the gauge reads 5-6 psi but no fuel cut.

Any ideas? has anyone had the pressure sensor go bad?

Check for leaks in the tubing to the boost guage. I had a cracked line myself and couldnt figure out why the boost was fluttering so bad!

Can you try hooking up my scan tool? See what the injector pulse width and boost/vacuum readings look like when the problem happens?

It might not be fuel cut. Maybe you're having some kind of ignition problem.

No, it feels like boost cut. I haven't made your scan tool yet so I can't connect it.

In 4th or 5th gear when its cold, I can count 1, 2, 3 then it cuts. When its warmer it wont cut. All signs point to boost cut, but I'll see. I'm gonna put some teflon tape on the gauge fittings soon and it should narrow it down. Everything on my ignition is new except for the coil pack, but no matter where I'm giving it full throttle (in the RPM band) it will cut after 3 seconds (only when its cold out)

Okay, yeah, the 3-seconds thing clinches it. It's almost certainly fuel cut.

It might help diagnosis to know what the ECU's seeing as far as boost pressure.

Many MBCs do raise boost when its gets cold, so you may in fact just be overboosting.

Did you originally set your boost pressure to be above 6 psi?

Yeah. I have no doubt from air density I'm overboosting. Its just that my boost gauge still says 6 psi. I'm wondering if anyone's ever had a pressure solenoid malfunction so it read higher than it was.

Sounds like FCD time to me.....

If the boost controller is a mechanical unit (i.e. run a hose from one of the vacuum runners), then the pressure solenoid would not effect it at all.

It reads "real" boost as seen by the manifold. It's not an electronic boost gauge that reads from the ECU, or directly from the boost control solenoid on the strut tower.

It's most certaintly fuel cut. My car started doing that, then started idling really high, then would stall constantly while in neutral. That was a fault of my IAC valve though. I was also stuck in 'limp mode' where the stock boost control would only keep the car at 5.5-6psi (VF11 wastegate boost). Good lord, the car was slow then, lol.

My stock boost control is completely looped off to itself. The pressure solenoid is still connected, and my gauge is T'd off the intake there.

Right, but the boost gauge isn't reading from that, and the solenoid isn't touching the turbo, therefore it's not controlling boost. So a problem with that would not manifest itself with lowered boost, or fuel cut.

I'm not sure what you're trying to say. The pressure solenoid can't control boost (since its not connected to anything) but if the pressure sensor is detecting overboost it can apply fuel cut.

I'm just wondering if its possible for the pressure sensor to be malfunctioning, or if my boost gauge is malfunctioning (by giving me a low reading).

Oh, I thought you were saying that you thought your boost gauge was reading incorrectly because of the control solenoid.

Overboost fuel cut hits at ~14psi at sea level. Hook up the scan tool and see what the ECU sees.

If it is seeing under 14psi, it could be detonating and just not throwing a code, or it could be IAC related or something like that.

I thought the ecu cuts fuel when it sees a sustained combination of atmospheric pressure combined with boost giving a certain reading. If that was the case, a well working pressure sensor would have fuel cut at around 13 psi or so around sea level, but if the preassure sensor was off, and giving more pressure, it would cut fuel earlier, right?

I'm not sure how that comes into play with a manual boost controller, though.